The present invention relates to an injection device of the type that receives a syringe, extends it, discharges its contents and then retracts it automatically. Devices of this general description are shown in WO 95/35126 and EP-A-0 516 473 and tend to employ a drive spring and some form of release mechanism that releases the syringe from the influence of the drive spring once its contents are supposed to have been discharged, to allow it to be retracted by a return spring.
Because of the stack-up of tolerances of the various components of the device, a certain margin of safety must be built into the activation of the release mechanism, to ensure that it is effective. The consequence of underestimating the safety margin is that the release mechanism may fail to operate even once the syringe contents have been discharged, which is unsatisfactory in a device that is supposed to retract automatically, particularly for self-administered drugs. On the other hand, overestimating the safety margin may mean that some of the syringe contents are discharged after the syringe has retracted, which results firstly in a short dose and secondly in what may be termed a “wet” injection. Wet injections are undesirable for the squeamish, particularly in connection with self-administered drugs.
UK patent applications nos. 0210123, 0229384 and 0325596 describe a series of injection devices designed to deal with this problem. Each makes use of a neat trick that delays the release of the syringe for a certain period of time after the release mechanism has been activated, in an attempt to ensure that the syringe has been completely discharged. The devices illustrated in UK patent application no. 0325596 make use of a two-part drive incorporating a fluid-damped delay mechanism that is particularly effective in ensuring complete discharge of the syringe contents. In each case, the device relies upon two unlatching mechanisms. The first unlatching mechanism initiates the fluid damping mechanism and the second releases the syringe from the actuator, allowing it to be withdrawn. The unlatching mechanisms are activated by components of the injection device having been advanced to nominal unlatching positions relative to the device casework.
A device 10 of this general character is illustrated schematically in FIG. 1. The sequence of operation is as follows. Firstly, the device 10 is armed. The user presses a release button and the syringe 14 is advanced a distance d1 by a drive spring 30, thereby compressing the retraction spring 26. This movement inserts the needle 18 into the patient. The plunger 23 is advanced a distance d2 by the drive spring 30, injecting most of the dose. Once nearly the entire dose has been injected, the first unlatching mechanism is activated, an operation illustrated schematically by the coincidence of components 1 and 3. The plunger 23 is then advanced a further distance d3 by the drive spring 30, injecting the rest of the dose. Finally, the second unlatching mechanism is activated, an operation illustrated schematically by the coincidence of components 2 and 4, and the retraction spring 26 then causes the needle 18 to be retracted by the distance d1.
Since the drive spring acts upon the same component of the device throughout, here referred to as the “actuator”, the distance that component must move between the device being armed and the second unlatching mechanism being activated is, subject to tolerance stack-up, equal to the sum of d1, d2 and d3. In the devices described in the applications mentioned above, all of this movement takes place to the rear of the syringe, which means that the overall length of the device must be greater than the sum of the length of the actuator, the distances d1, d2 and d3 and the length of the syringe body not including the needle.
The best design of injection device is one that is compact. This is important both to the ergonomics of the device and to its manufactured cost. The length of the device can be reduced by allowing the actuator to move past the syringe, and by having the unlatching mechanisms activated in front of the syringe. However, this would require the actuator and its unlatching mechanisms to pass around the space occupied by the syringe, involving an increase in diameter of the device that negates the length savings.